Mobile homes are being marketed as a downsizing option for retirement. Simply release the equity in your bricks-and-mortar house and move to a rural location surrounded by like-minded people.

But an investigation by BBC Radio 4's Face the Facts has found the law protecting people living on mobile home parks is weak. And a change to weed out criminals from moving into the industry is still awaited nearly a decade after it was first suggested.

On one park, the Glen in Worcestershire, elderly residents suffered from a series of terrifying arson attacks (pictured left).

One woman lived almost next door to one of the homes burnt to the ground: "When I saw the blaze I opened my door and fell down the steps, because I thought: 'It's going up in a minute.' Of course I was thinking: 'If they put anything under mine I won't stand a chance.' "

She and three others living on the site were eventually blackmailed into selling their homes to the owners for only £1.

A police investigation resulted in seven men, including site owners John and Simey Doherty, getting jail sentences of 64 years in total.

The secretary of the all-party parliamentary group for park homes, Lord Graham of Edmonton, has been pushing, since 2000, for a fit-and-proper person test to be introduced for a site owner to get a licence. "The sad fact is that many people living on park homes are elderly, disabled and frail. Defenceless people are being preyed upon by unscrupulous site owners," he said.

In a case last month, which didn't involve harassment and intimidation, the owners of nine mobile home parks – brothers Sydney and Nevada Smith – were convicted of money laundering. They had £1.5m confiscated.

Lord Graham wants anyone with a criminal conviction or county court appearances to be scrutinised under a new licensing system. Residents on a site bought by Maurice Sines in 2001 – Ladycroft Park in Blewbury, Oxfordshire – went to county court in 2005 after suffering harassment and intimidation and being unable to sell their homes. Site owners can approve the buyers of homes but are supposed to give a valid reason for withholding that approval. Some owners abuse this.

Sheila Austin, who led the court action, said: "He blocked sales saying homes would be dragged around the park, and threatening and shouting abuse at people. It was horrendous."

Thirty people sold their homes to Sines. One valued at £50,000 was sold for £10,000. They were then demolished and replaced with new ones. The 25 remaining residents got £75,000 damages in an out-of-court settlement.

Sines signed an order not to threaten, abuse or harass residents, members of their families, visitors or prospective purchasers, and not to block sales. He has now sold the site. Sheila and the others had help from the villagers of Blewbury who raised £65,000 for the court case. People on another mobile home park owned by Sines, and his business partner James Crickmore, at Hardwick Bridge in Norfolk, weren't so fortunate.

Jackie, not her real name, said: "They were banging on the doors in the early hours of the morning saying: 'You have got to get out or we'll hook your van up and pull it round the site until it just falls to bits.' It was unbelievable."

Site owners can buy new homes from the manufacturers for around £60,000. Once on site, the home can be sold for up to £160,000, so site owners have an incentive to clear their parks of older property.

Face the Facts also spoke to residents on other parks owned by Sines and Crickmore and found complaints that sales were being blocked.

Sines and Crickmore deny the allegations and insist they don't block sales on any of their sites. They say homes they bought were not fit to be sold on the open market. Instead, they were trying to improve parks such as Hardwick Bridge and Ladycroft. And they deny harassing or intimidating residents, instead saying residents were harassing them.

Sines agreed to an order not to harass and intimidate residents at Ladycroft Park because he wasn't doing it in the first place, he says. And, as for the £75,000 damages, he claims that was to compensate people for living in a building site for three years.

The government has now finished consulting on the idea of bringing in a fit-and-proper person test for site owners, but it may still be many years before it becomes law.